1. Field of the Invention
The present invention relates to an actuator, and more particularly to an actuator which is free from electrostatic charge and from effects of external noises.
2. Description of the Related Art
Stepping motors are conveniently and well controllable and are used in various audio visual and office automation equipments. Especially, a permanent magnet (PM) stepping motor is low in cost, and therefore, recently, part of the PM stepping motor is utilized to structure an actuator, in which the rotary motion of a rotor is converted into the linear motion of an output shaft (hereinafter such an actuator is referred to as a “PM motor type actuator”). In the PM motor type actuator, a rotary-to-linear motion mechanism, which is not found in a motor structure, is provided between an output shaft (corresponding to a rotary shaft of a motor) and a mold resin portion as a sleeve having a magnet fixed thereon, whereby the rotary motion of the magnet and the mold resin portion is converted into the linear motion of the output shaft. Such an actuator is used to activate a headlamp optical axis control unit in a vehicle, a gas flow amount control valve, and the like.
FIG. 7 is a cross sectional view of one type of PM motor type actuator 100 (refer to, for example, Japanese Patent Application Laid-Open No. 2002-122203). The actuator 100 generally comprises a stator assembly 101, a rotor unit 110, an output shaft 113, and a housing 118.
The stator assembly 101 includes stator units 101a and 101b coupled to each other. The stator unit 101a includes outer and inner stator yokes 102a and 103a made of a soft-magnetic steel plate and having a plurality of pole teeth 105a arranged in a comb-like configuration, and a coil 104a disposed between the outer and stator yokes 102a and 103a. In the same way, the stator unit 101b includes outer and inner stator yokes 102b and 103b with a plurality of pole teeth 105b, and a coil 104b. The first stator unit 101a is stacked on the second stator unit 101b, and the first and the second stator units 101a and 101b thus combined are fixedly coupled together by resin injection molding. A front plate 117 having, at its outer circumference, claws for catching the housing 118 is attached to the first stator unit 101a, thus completing the stator assembly 101.
The rotor unit 110 includes a magnet 111, and a mold resin portion 120 having a ring-like nut portion 112. The magnet 111 is shaped into a hollow cylinder, fixedly attached onto the mold resin portion 120, and is disposed inside the stator assembly 101 so as to oppose the pole teeth 105a and 105b of the stator yokes 102a, 103a, 102b and 103b with a gap therebetween. The mold resin portion 120 is formed into a cylindrical configuration by resin injection molding, has the nut portion 112 disposed therein, and has the magnet 111 disposed at its outer circumference. The both axial ends of the mold resin portion 120 are rotatably supported by respective bearings fitted to the housing 118 and a rear cap.
The output shaft 113 has a head 116 attached at one end portion thereof exposed, has a screw portion (male screw) 114 formed on the other end portion thereof enclosed, and has a rotation stopper pin 115 inserted through the middle portion thereof orthogonally thereto and engaging with axially extending grooves formed inside the housing 118 so as to be axially movable and circumferentially immovable.
A rotary-to-linear motion mechanism comprises the nut portion 112 of the rotor unit 110 and the screw portion 114 of the output shaft 113, wherein the nut portion 112 engages threadedly with the screw portion 114 so that the rotary motion of the rotor unit 110 is converted into the axial motion of the output shaft 113.
The housing 118 has the aforementioned axially extending grooves for accommodating the rotation stopper pin 115 and is structured so as to appropriately position the stator assembly 101 and the rotor unit 110. The rotor unit 110 is put in the stator assembly 101, then the housing 118 is put over the stator assembly 101 and the rotor unit 110, and the claws of the front plate 117 are bent up so as to rigidly catch the housing 117.
The actuator 100 structured above operates as follows. When current is applied to the coils 104a and 104b, the rotor unit 110 is caused to rotate by magnetic attraction and repulsion generated between the coils 104a and 104b and the magnet 111, and the output shaft 113, which is prohibited from rotating by the rotation stopper pin 115 accommodated in the grooves of the housing 118, is caused to linearly move in the axial direction by the rotary-to-linear motion mechanism, specifically the nut portion 112 and the screw portion 114 threadedly engaging each other.
Recently, reduction in dimension, weight and also cost is increasingly requested, and in order to meet the request, an engineering plastic which is favorable for weight and cost reduction is often used for the mold resin portion 120 of the rotor unit 110, the head 116 attached to the tip end of the output shaft 113, the resin for injection molding of the stator assembly 101, and the like. An actuator using an engineering plastic as described above is apt to be electrostatically charged, and electronic parts may possibly suffer electrostatic destruction. Also, importance is being put on electromagnetic environment in equipments and systems, and noise issues, such as EMC and EMI must be carefully considered.
If an actuator is attached by metallic screws to a metallic chassis of an equipment, then static electricity and electromagnetic noise generated in the actuator are caused to flow, via the metallic screws, easily into a chassis grounded, thus preventing electrostatic charge and electromagnetic noise problem.
On the other hand, recently, the chassis of the equipment to which the actuator is attached is often made of an engineering plastic for reduction in weight and cost, and also for better productivity. The engineering plastic is light and strong but not electrically conductive therefore failing to conduct static electricity and electromagnetic noise generated in the actuator into the chassis.
This problem is solved by providing a grounding pattern used in, for example, a molded motor. Specifically, a grounding pattern formed on a printed board is made electrically continuous with a stator via screws made of an electrically conductive material, and a lead wire as a grounding wire is connected to the grounding pattern (refer to, for example, Japanese Patent Application Laid-Open No. H07-087696).
Also, in case of a solenoid valve on a vehicle, one end of a conductive clamp is welded to the outer circumference of a body case, and the other end thereof is crimped onto a grounding terminal (refer to, for example, Japanese Utility Model Application Laid-Open No. 06-028448).
The above solutions, however, have the following problems.
In the solution described with reference to the Japanese Patent Application Laid-Open No. H07-087696, the grounding pattern must be formed on the printed board, and an appropriate structure is required for duly connecting the grounding pattern to the stator by the conductive screws. Also, a screwing-up process is required thus deteriorating workability.
In the solution described with reference to the Japanese Utility Model Application Laid-Open No. H06-028448, the welding and crimping works result in deteriorated workability.